1. Field of the Invention
The present invention generally relates to structures in which a volume between two substrates forming a panel is filled with a liquid. More particularly, this invention relates to liquid crystal displays of the type used with computer equipment, and provides a method and device for maintaining a desired level of liquid crystal material between two substrates forming a display panel so that bubble formation does not occur as a result of changes in volume of the liquid and/or the cavity between the substrates, as would occur as a result of temperature fluctuations.
2. Description of the Prior Art
Liquid crystal display (LCD) panels are made in a variety of ways, including flat panel displays used in portable computers. Flat panel displays typically comprise two flat substrates spaced apart to define a cavity therebetween in which a liquid crystal (LC) material is contained. Stringent requirements must generally be placed on the quality of displays used in the computer industry in order to find acceptance in the marketplace. For example, small defects in an LCD can visually be very disturbing and easily detected by the viewer. One known problem with flat panel displays is associated with temperature fluctuations, which can be quite considerable. For example, a computer and its flat panel display may be stored at temperatures well below xe2x88x9220xc2x0 C., then utilized in an environment where the display may operate at a temperature of 40xc2x0 C. or more. With such fluctuations, the LC material and the cavity in which the LC material is contained will expand and contract. Because liquid display materials have a greater coefficient of thermal expansion than the substrates of the display, the excessive contraction of the LC material relative to its cavity can have the effect of creating vacuum spaces or bubbles in the LC material. If there is undue expansion, the problem is less severe though excessive volume expansion of the LC material may damage the cavity or cause poor definition due to the ensuing incomplete polarization of the panel light upon switching of individual pixels.
The expansion/contraction problem is especially likely to occur with the so-called one drop fill (ODF) method of the type disclosed in U.S. Pat. No. 5,263,888 to Ishihara et al., in which relatively rigid spacers are used to separate two flat substrates. Such spacers cannot be readily compressed to allow the substrates to move toward each other, which would decrease the volume of the cavity containing the LC liquid to compensate for a decrease in the volume of the LC material as a result of a temperature drop.
From the above, it can be seen that it would be desirable to provide a solution to the problem of visual defects that occur in a LCD flat panel display as a result of fluctuations in the temperature of the display.
The present invention provides a device and method for maintaining the volume of a liquid contained within a cavity between two substrates to something equal or nearly equal to that of the volume of the cavity. In this manner, the formation of voids or bubbles within the liquid is substantially or completely eliminated or minimized. If the liquid is a liquid crystal (LC) material between two flat display substrates, the invention essentially eliminates thermally-induced defects that would otherwise be visible to the user.
The present invention generally provides means for minimizing the volume differential between an LC material and a display panel cavity in which the LC material is contained. One approach provided by the invention is to deliver additional LC material to the cavity and receive LC material from the cavity as required in response to changes in temperature. For this purpose, a reservoir is provided from which the LC material is dispensed and accumulated as required to compensate for the differential volume expansion of the LC material and display cavity. Means are preferably provided for reducing the volume of the reservoir in response to a temperature drop, thus forcing some of the LC material within the reservoir into the panel cavity to compensate for the contraction of the LC material within the cavity that occurred as a result of the temperature drop. Similarly, if there is an expansion of the LC material within the panel cavity, the means associated with the reservoir allows the excess volume of LC material to flow from the panel cavity into the reservoir. Another approach of the invention is to provide a panel cavity with a controllable variable volume. With either embodiment, though the ambient temperature of the display fluctuates, the panel cavity remains substantially filled with the LC material, so that voids and bubbles are avoided.
Other objects and advantages of this invention will be better appreciated from the following detailed description.